The present invention relates to a method of manufacturing a polyimide thin film and to a method of manufacturing a liquid crystal orientation film of polyimide. 2. Description of the Related Art
Recently, very thin films of organic materials having a thickness of about 100 nm attract attentions in this technical field. The thin films of this type are mainly insulating thin films such as insulating protective films for semiconductor devices, dielectric thin films for capacitors and liquid crystal orientation films.
Thin films can be formed by, for example, a wet process or a dry process. It is known to the art that, when it comes to the insulating protective films for semiconductor devices, polyimide films are formed by the wet process. However, it is said to be desirable to form the polyimide films by the dry process. In general, it is difficult to remove dust in the case of the wet process. In addition, the wet process is incompatible in many cases with the process of manufacturing semiconductor devices because the dry process is mainly employed for the manufacture of the semiconductor devices. When it comes to the formation of a liquid crystal orientation film which permits orientation of liquid crystal molecules, it is known to the art to employ an oblique vapor deposition of inorganic materials. It is also known that a coating of a silane coupling agent or an organic high molecular weight compound is formed first, followed by rubbing the coating. However, the cell gap is small in a liquid crystal element using a ferroelectric liquid crystal material. It follows that, if the orientation film is nonuniform in thickness or dust is attached to the film formed by the wet process, the orientation of the liquid crystal molecule is rendered poor, leading to decisive defects in the characteristics of the liquid crystal element.
As described above, the wet process generally gives rise to a difficulty in terms of the dust removal. Further, the specific techniques falling within the scope of the wet process give rise to individual problems inherent therein. To be more specific, a solvent casting method is one of the most popular techniques of the wet process for forming a thin film. It is known to the art that the spin coating method, i.e., a kind of the solvent casting method, permits forming a thin film of about 10 nm in thickness depending on the kind of the polymer used. In this case, however, it is difficult to form a thin film having a uniform and dense structure and a high insulating property.
Also, attentions are attracted by the technique of forming an LB film of an organic material on the surface of, for example, a semiconductor or metal substrate for allowing the LB film to perform the function of an insulating film. Recently, vigorous researches are being made on the technique of improving the heat resistance and the mechanical strength of the LB film. For example, it is reported in "Electronics Letters, 20, No. 12, p. 489 (1984)" that a condensed cyclic compound excellent in heat resistance is incorporated in an organic composition used for forming a thin film. Also, the use of a low molecular weight compound, which is incapable of polymerization, in combination with a high molecular weight compound is reported in "Journal of Colloid and Interface Science, 79, p. 268 (1981)". Further, the idea of forming a film of a low molecular weight compound capable of polymerization, followed by the polymerization treatment, is reported in "Thin Solid Films, 99, p. 249 (1983)". Further, a film formation from a vinyl polymer is reported in "Journal of Colloid and Interface Science, 131, p. 218 (1989)". However, the LB film formed by any of these prior arts fails to exhibit sufficient heat resistance and mechanical strength.
Recently, a method of forming a polyimide film by the LB method is reported in, for example, "Polymer Preprints, Japan Vol. 36, No. 10, p. 3215 (1987)". It should be noted that polyimide exhibits the highest heat resistance and the highest mechanical strength among the organic materials. However, in order to convert a polyimide precursor thin film into a polyimide thin film, a treatment with chemicals is required as described in "Thin Solid Films, 160, p. 15 (1988)", or a heat treatment at such a high temperature as about 300.degree. C. is required as described in "Thin Solid Films, 160, p. 21 (1988)". The use of a polyimide film formed by the LB method is also reported with respect to a liquid crystal orientation film in, for example, Published Unexamined Japanese Patent Application No. 63-52119. Rubbing need not be employed in this technique. In addition, it is possible to form a very thin film, making it possible to eliminate the coloring problem caused by polyimide. In this technique, however, it is also necessary to employ a treatment with chemicals or a heat treatment. What should be noted is that such a treatment possibly brings about undesirable results depending on the element in which said polyimide film is used.
The LB method is featured in that a monomolecular film formed on the water surface is accumulated on a solid substrate. In order to use an LB film as an electrically insulating thin film, it is necessary for the film to have a thickness of at least 100 nm so as to prevent the flow of a tunnel current. Naturally, it is necessary to superpose many LB films one upon the other, leading to a long manufacturing time of the insulating film. The LB method is also defective in that the kinds of molecules used for forming the monomolecular film are much restricted.
A water surface spreading method, which resembles the LB method, is also known as a method of forming a thin film. For example, polymers having a flexible molecular chain and soluble in an organic solvent such as polyvinylidene fluoride copolymer, polystyrene and polysiloxane do not form a monomolecular film on the water surface, but are known to form a thin film about 10 nm thick (see, for example, "Polymer Preprints, Japan Vol. 36, No. 10, p. 2250 (1987)" and "Polymer preprints, Japan Vol. 38, No. 12, p. 884 (1989)"). However, many heat-resistant polymers such as polyimide are insoluble in general in an organic solvent, making it impossible to apply the water surface spreading method to these polymers. Certainly, some polyimides having a particular molecular structure are soluble in an organic solvent. However, since polyimide has a rigid molecular chain and a strong force of association among molecules, the polymer molecules fail to be spread sufficiently on the water surface. It follows that a very thin film cannot be formed in many cases.
On the other hand, it is also possible to employ a dry process such as a vacuum vapor deposition for forming a thin polymer film. In order to solve the problem of dust attachment which is inherent in the wet process, it is most desirable to employ a dry process utilizing a vacuum system. As a matter of fact, thin films of polyethylene or polytetrafluoroethylene are actually manufactured by the vacuum vapor deposition method. However, it is generally difficult to employ the vacuum vapor deposition for forming a heat resistant film such as a polyimide film. To overcome the difficulty, it is proposed to perform vapor deposition of two kinds of monomers for polymerization on a substrate so as to prepare polyimide (see, for example, "Polymer Preprints, Japan Vol. 37, p. 286 (1988)"). In this proposal, however, it is difficult to set the optimum conditions for the vapor deposition. Also, it is difficult to obtain polyimide having a sufficiently large molecular weight.